1. Field of the Invention
This present invention relates to chemical decontamination and more particularly refers to a new and improved method for the chemical decontamination of parts, components, subsystems and systems, particularly of water-cooled nuclear reactors, in three stages.
2. Description of the Prior Art
In the primary loop of a nuclear power plant, a coherent, dense oxide layer is generated after a short period of operation due to the corrosion of the structural materials. This initially inactive oxide layer becomes contaminated, i.e. radioactive, during the operation. This applies to components subjected to direct radiation and also to components which are not in the direct radiation field of the core region. This contamination is caused by the incorporation of activated corrosion products into the oxide layer of the structural materials. As the process runs continuously, it leads to an enrichment, particularly of the long-life nuclides, in the oxide layer. The need is therefore urgent to find procedures for removing this surface contamination by suitable decontamination means. This requirement becomes more and more urgent with increasing operating time, as the maintenance of the systems as well as particularly repair work leads to increasingly higher radiation exposures of the operating personnel.
It has heretofore been attempted to decontaminate contaminated surfaces by means of aqueous solutions of mineral and organic acids. The results obtained thereby, however, were entirely unsatisfactory, especially since at the same time damage to the structural material was registered. Only the two-stage APAC (Alkaline Permanganate Ammonia Citrate) method showed good decontamination factors, but it likewise led to selective corrosion phenomena and to excessively strong attacks on the base metal.
In addition, the decontamination solution with the APAC method is inhibited by sulfur-containing substances. However, sulfur compounds are prohibited in primary loops of water-cooled nuclear reactors, as sulfur can lead, in the case of Ni-alloys, to selective corrosion phenomena in later operation.